Spline to Position Track
This converts a spline into an animation path. First, select the object whose animation path you want to change. Next, activate the Spline to Position Track command. Based on the first Spline selected in the Object Manager, Cinema 4D will create new X, Y and Z position tracks for the object so that it follows the path of the spline.
Make sure you are not in Link TL/OM Selection mode. Otherwise it will not be possible to select objects and splines separately.